Flat panel displays (FPDs), including active-matrix liquid crystal displays (AMLCDs) and active-matrix organic light emitting diode (AMOLED) displays, are becoming vital in modern information exchange and communication processes. Thin-film transistors (TFTs) are the essential active devices of the active-matrix backplane. The TFTs control the on/off switching of the light for each pixel of a FPD. The resolution of an FPD is characterized by the number of pixels per inch (ppi). A higher ppi value implies smaller pixel size and thus a smaller TFT size per pixel. The preferred ppi highly depends on the specific application of the FPD. For example, modern smartphones usually require a ppi greater than 300 for both AMLCD and AMOLED panels. For near-eye applications, such as augmented reality (AR) and virtual reality (VR) application, a ppi greater than 500 ppi (and up to about 1000 ppi) is theoretically preferred to prevent a grainy effect.
The downscaling of the TFT pixel becomes quite challenging for FPDs with pixel densities beyond 600 ppi due to several hindering factors. In particular, the resolution of the photolithography tools used to form the TFT pixel determines the minimum line width and space in TFT panels, which thereby controls the maximum ppi for a FPD. The smallest resolution of photolithography tools in the current FPD industry is about 2 μm, which is much poorer than that in integrated circuit (IC) industry. Accordingly, the pixel area of the FPDs is restricted by the 2 μm resolution of the photolithography tools. In addition, the driving capability of a TFT is determined by its mobility and geometric size. For lower mobility, a larger TFT, more specifically a larger width to length (W/L) ratio, is required to compensate the driving current, thus enlarging the pixel area. Further, for OLED panels, the light emitting uniformity is directly related to the uniformity of TFTs. Considering the unsatisfied uniformity of incumbent TFTs, compensation circuits are currently essential in the pixel of AMOLEDs, inevitably increasing the TFT numbers, connection lines and contact holes, and thus degrading the ppi. Furthermore, a storage capacitor is required in the pixel to hold the charges during the whole frame period. The area of the capacitor is closely related to the leakage current of TFT. A leaky TFT required a larger area capacitor. Accordingly, techniques for increasing the pixel densities of FPDs while accounting for these various hindering factors are highly desired.